In this case the best thing to do will be to purchase a 2.5 SSD, and install it on the NUC on the M.2 port, then, transfer all the information from the embedded memory to the SSD.

Also, once you get the SSD and transfer all the information to it, you can download and install CCleaner or another cleaning tool to free up some space and to be able to have more than 20GB of memory free, to do the update.